Th objective of this proposal is the examination of human parainfluenza virus 3 (HPIV3) RNA sequences involved in genome transcription and replication. We know little about the cisacting RNA sequences involved in HPIV3 RNA replication and transcription. However, our limited knowledge of HPIV3 and analogy with other members of the paramyxovirus family suggests terminal sequences are important for transcription and replication and intergenic junctions are critical in the control of transcription. Similarly, the mechanism mediating the switch between the transcription and replication phases of the replication cycle is only vaguely understood. By mutating nucleotides present in the leader and trailer regions of the vRNA I hope to detemine what RNA sequences are critical for RNA replication, whether RNA secondary structure is important, and whether the degree of complentarity between the termini of HPIV3 RNA influences either replication or transcription. The specific aims are as follows: Aim 1: Construction of an HPIV3 replicon Aim 2: Map cis-acting RNA elements with respect to RNA replication and transcription Aim 3: Construction of an infectious HPIV3 clone and assay of Aim 2 mutations.